1. Field of the Invention
The present invention relates to ethylene oligomerization to produce alpha-olefins, and in particular, to a heterogeneous catalyst and a process for ethylene oligomerization.
2. Description of the Related Art
An oligomer is a multiple unit of a monomer, and in this case the monomer of interest is ethylene. Oligomers include dimers, trimers, tetramers, and so on, but the resulting molecule is smaller than a polymer. Oligomers of ethylene are linear alpha-olefins which typically contain between 4 and 20 carbon atoms. Linear alpha-olefins are commercially important, and thus, improvements in the production of linear alpha-olefins are highly desirable.
In U.S. Pat. No. 3,676,523, issued to Mason on Jul. 11, 1972, Mason discloses a process of oligomerizing ethylene to linear alpha-olefins by reacting ethylene in a liquid phase solution in the presence of a nickel catalyst. Mason describes reacting nickel chloride hexahydrate, sodium borohydride and a potassium salt of a dihydrocarbylphosphinobenzoic acid, such as o-diphenylphosphinobenzoic acid, in a reaction medium of 1,4-butanediol and ethylene. Thus, Mason discloses a homogeneous catalyst and a process of oligomerizing ethylene in the presence of the catalyst in a polar organic solvent. The process is characterized by ethylene conversion to a linear alpha olefin product mixture of relatively high proportion of olefinic products in a higher molecular range including C.sub.12 -C.sub.20 alpha-olefins.
Lutz, the inventor of U.S. Pat. No. 4,528,416 issued Jul. 9, 1985, disclosed a process for producing ethylene oligomers in a polar organic solvent in the presence of a catalyst which is a chelate of nickel with a bidentate ligand. The solvent mixture contained between about 40 and 82% by weight of an aliphatic dihydric alcohol and between about 18 and 60% by weight of an aliphatic monohydric alcohol. It is critical to the Lutz process that the ethylene partial pressure be at least about 800 psig. Lutz stated that the use of the described reaction solvent mixture resulted in improvement in the rate of ethylene oligomerization and/or modification of the carbon number distribution of the product oligomer mixture. Lutz states that it is essential that the catalyst composition be prepared in the presence of ethylene, and the ethylene should be at a substantially elevated pressure, preferably in the range from 400 to 1,500 psig. The catalyst of Lutz is homogeneous, and it is necessary to carry out the oligomerization process in the solvent described above.
Wu in U.S. Pat. No. 5,286,696, issued Feb. 15, 1994, describes a heterogeneous catalyst. Wu states that homogeneous catalyst systems, such as described above, are not suitable for continuous processes or are relatively expensive. Wu further states that the ethylene oligomerization processes known at that time had not always achieved high catalyst productivity and good product selectivity. Wu discloses an ethylene oligomerization process comprising: (1) combining a nickel compound and a phosphine compound and a solvent to form a mixture; (2) combining this mixture with a phosphated alumina under an ethylene pressure to form a catalyst system; (3) contacting ethylene with the catalyst system under oligomerization conditions to produce higher olefins having more than 2 carbon atoms; and (4) recovering the higher olefins. Wu states that an advantage of his invention is the use of a heterogeneous composition that is easily employed in a continuous process for ethylene oligomerization.
Thus, a heterogeneous catalyst system, such as where the catalyst is a solid suspended in a liquid diluent, is advantageous for a continuous process, which can have a higher production rate than a batch process. Further, it remains true that even slight improvements in the process and/or catalyst for making ethylene oligomers is highly desirable because there is a significant commercial demand for linear alpha-olefins, such as produced by ethylene oligomerization.